It's been awhile since I've handled any wheel spacers. I've seen some that are made of soft metal (aluminum, zinc, etc.) and aren't hardened. That means they will compress slightly when the lug nuts are tightened. The compression will happen slowly. The lug nuts may torque just fine when you install them, but as the spacer compresses, the lug nuts will exert less tension on the studs. It's the clamping force between the wheel and the hub that transfers the torque between them. So, if the nuts are loose, the wheel will start to move around slightly. That can lead to failures of the studs.
Steel spacers are probably OK. And, hub centric spacers that won't move around between the wheel and the hub would be better.
And, you really have to careful of thread engagement when using spacers as
@Left Coast Geek states. The ability of the lug nut and stud to keep the wheel on the vehicle is related to the amount of thread engagement in the lug nuts.